The present invention relates to an energy storage system balancing device arranged to perform balance processing of charge states of a plurality of energy storage systems being disconnected from each other and from at least one external load. The present invention also relates to a vehicle system. It further relates to a method for performing said balance processing.
Chargeable and dischargeable electric energy storage systems, normally called batteries, are used in many applications. Since the introduction of hybrid and electrical vehicles, the need has increased for optimizing the batteries in order to be able to optimize charging, storage and use of the electrical energy.
The battery modules each have a plurality of battery cells (electric cells) connected in series and/or in parallel. Charge-discharge characteristics ma vary among the plurality of battery cells. Therefore, it is preferable that charge-discharge of each battery cell is individually controlled in order to balance the cells within the battery to prevent the battery cell from being over-charged or over-discharged.
Consequently, each battery comprises at least one equalization system arranged to balance the cells within the battery. The number of equalization systems may vary in dependency of the number of cells, if the battery comprises battery modules mounted together to form the battery, and if one equalization system is used to control one or a plurality of cells. Despite the number of equalization systems, all the cells of the battery is monitored and managed by said systems to ensure an optimization of the battery over time and in capacity.
US 2011/0127963 disclose an equalization device. The device is arranged to perform equalization processing of charge states of a plurality of battery cells that are connected in series, wherein the plurality of battery cells are divided into a plurality of groups. The equalization device includes a detector arranged to detect the charge states of the plurality of battery cells, a selector arranged to select any of the plurality of groups based on the charge states detected by the detector, and an equalization processing unit arranged to perform the equalization processing by selectively causing the battery cells that belong to the group selected by the selector to discharge.
When connecting two or more batteries in parallel, it is important that they are at the same state of charge (SOC) in order to have a uniform distribution of power and equal life time. Once the batteries are connected to each other they will strive to reach the same voltage level since they are physically connected to each other. This is a wanted feature since they will, if the time is long enough, reach the same pack voltage and by that the same SOC level permitted that the temperature is kept constant. By this the batteries in the system will try to share the workload and distribute the energy evenly.
There is then a challenge to ensure that the State Of Charge (SOC) for the different batteries is the same in order to have a uniform distribution of power and equal life time. If the batteries have different SOC at the time of connection to each other they will not automatically converge to the same SOC even though they stabilize to the same voltage level. This will also result in that the life time differs between the batteries. If the batteries are used at different SOC caused by unbalanced batteries, there is a risk that one battery will be used excessively and hence have an accelerated aging.
It is desirable therefore to ensure so that the charge state is the same for a plurality of batteries being connected to each other.
According to an aspect of the present invention, an energy storage system balancing, device arranged to perform balance processing of charge states of a plurality of energy storage systems being disconnected from each other and from external loads. Each of said storage systems is divided into a plurality of cells and at least one equalization device is arranged in each storage system for balancing the cells within said system. Said balancing device is arranged to at least collect information about voltage of at least one cell in each storage system from the equalization device/-s and determining a balancing status of the storage systems on the basis of the collected information. Said balancing device is further arranged to determine a target cell voltage in dependency of the balancing status and provide the target voltage to each equalization device to control said balancing of the cells.
According to another aspect of the invention, a vehicle system is provided comprising the energy storage system balancing device.
According to another aspect of the invention, a method for balance processing of charge states of a plurality of energy storage systems being disconnected from each other and from external loads. Each of said storage system is divided into a plurality of cells and at least one equalization device is arranged in each storage system for balancing the cells within said system. The balancing device performs the steps of:                at least collecting information about voltage of at least one cell in each storage system from the equalization device/-s,        determining a balancing status of the storage systems on the basis of said collected information,        determining a target cell voltage in dependency of the balancing status,        providing the target voltage to each equalization device to control said balancing of the cells.        
With aspects of the present invention, the batteries are controlled to be in the same charge state region, which means that they will have the same power performance and energy performance. This also means that the batteries are used equally and will degrade simultaneously. The remaining capacity of the battery set will be known all the time and the lifetime will be optimized. Further, connecting batteries together instead of increasing cells in one battery will have more benefits.